Nanocrystalline materials are nano composites characterized by an ultrafine grain size (less than 50 nm). Nanoclusters are the subject of current interest due to their unusual optical, electronic, and magnetic properties which often differ from their bulk properties. The spatial confinement of electronic and vibrational excitations in nanoclusters result in a widening of the energy band gap and observation of quantum size effects. Quantum size effects and large surface to volume ratios can contribute to the unique properties of nanoclusters, which for example include a phenomena that when below a critical size the magnetic particles become a single magnetic domain and are superparamagnetic.
Although nanoclusters have received attention from both theoretical and experimental standpoints, the greatest challenge at the present time is to find out an effective synthesis procedure. The fundamental challenges in nanostructured materials include: ability to control the scale of the nanostructured system; ability to obtain the required composition with the controlled effects, concentration gradients, etc.; understanding the influence of the size of building blocks in nanostructured materials, as well as the influence of microstructure of the physical, chemical, and mechanical properties of this material; and transfer of developed technologies into industrial applications including the development of the industrial scale of synthesis methods of nanomaterials and nanostructured systems.
A number of methods of nanocluster fabrication have been developed which include Radio frequency plasma torch synthesis of γ-FeNx nanoclusters have been reported by Z. Turgut, et al. of Carnegie Mellon University. In their approach, a plasma gas mixture of argon and hydrogen were used as a sheath gas. Micron sized iron particles were injected into the plasma stream using argon as a carrier gas. Ammonia was used as a nitrogenization source. By controlling the injection rate, a mixture of 27 nm FeNx and 55 nm Fe powder was achieved.
Graphite encapsulated metal nanoclusters were reported to be synthesized by D. Lynn Johnson, et al. of Northwestern University using high temperature electric arc technique. Carbon and metals of interests were co-evaporated by producing an electric arc between a tungsten cathode and a graphite/metal composite anode. The encapsulation occurred in-situ. The powdered material collected consisted of GEM and bare metal nanocrystal as well as amorphous carbon particles.
PbS and CdS colloids of nanometer dimension have been reported to be synthesized by controlled precipitation of the metal sulfide in water and acetonitrile solution (H. J. Watzke, et al., Journal of Physical Chemistry, 91, 854, 1987). Although these colloids have shown quantum sized effects, they have a broad size distribution. Synthesis of nanoclusters other than CdS and ZnS has thus far been substantially unsuccessful.
CdS nanoclusters have been synthesized within the pore structure of the zeolite (Y. Wang, et al., Journal of Physical Chemistry, 91, 257, 1987). The coordination of Cd atoms with the framework of oxygen atoms of the double six ring windows of zeolite leads to formation of stable nanoclusters with the structural geometry superimposed by the matrix.
Metal nanoclusters have been prepared by the solution phase thermolysis of molecular precursor compounds (J. G. Brennan, et al., Chemical Materials, 2, 403, 1990), such as [Cd(SePh)2]2[Et2PCH2CH2PeT2].
Nanocluster of CdSe has been synthesized using organometallic reagents such as Se(TMS)2 in inverse micellar solution (A. P. Alivisatos, et al., Journal of Physical Chemistry, 90, 3463, 1989). Arrested precipitation in reverse miscelles gives a bare semiconductor lattice and in situ molecular modification of the cluster surface enables isolation of the molecular product with a variety of organic surface ligands.
Gold nanoclusters have been fabricated using a metal vapor deposition technique (J. K. Klabunde, et al., Chemical Material, 1, 481, 1989). In this method, gold vapor was codeposited with liquid styrene or methyl methacrylate (as vapor) at liquid nitrogen temperature.
The first successful attempt to use block copolymer to fabricate metal nanoclusters is believed to have been accomplished by Morkned, et al. (Applied Physics Letters, 64, 422, 1994). In this method, metal vapor was deposited on the surface of a microphase separated PS-PMMA diblock copolymer. After deposition, the film was annealed under vacuum for twenty-four hours. The resulting nanoclusters had a narrow size distribution. The shape and size of the nanoclusters were additionally fine tunable.
Recently, research at MIT (R. T. Clay, et al., Supra Molecular Science, 4, 113, 1997) and at the University of Maryland, College Park have synthesized metal nanoclusters inside the microphase separated domains of diblock copolymer. The self-assembled nature of domain structures permits good control over the shape and size of nanoclusters. Polymer matrix also provides kinetic hindrance to aggregation of nanoclusters of larger particles. Nanoclusters within block copolymer show 3-D ordering and furthermore the density of nanoclusters are high enough for synthesizing non-linear devices for commercial applications.
Metal nanoclusters of Cu, Ag, Pd, Pt, and binary metal oxide nanoclusters of Fe2O3 and CuO have been synthesized within microphase separated domains of diblock copolymers [Y. N. G. Scheong Chan, et al., Chemical Material, 4, 1992, 24, Y. N. G. Scheong Chen, et al., Journal of American Chemical Society, 114, 1992, 7295, Y. N. G. Scheong Chen, et al., Chemical Materials, 4, 1992, 885, and B. H. Sohn, Chemical Materials, 9, 1997, 113]. The self-assembled nature of the micro-domains permits control over the shape and size of the nanoclusters. The interfaces between the blocks of the diblock copolymers play an important role in the nucleation and growth of clusters and induces a narrow size distribution. The polymer matrix additionally provides schematic hindrance to aggregation of nanoclusters.
Cobalt ferrite, CoFe2O4, is a well-known hard magnetic material with high cubic magneto-crystalline anisotropy, high coercivity and moderate saturation magnetization. It would be highly desirable to provide room temperature synthesis of mixed metal oxide nanoclusters within a polymer matrix for obtaining diblock copolymer-CoFe2O4 nanocomposites with the needed magnetic properties while only single metal incorporation within a block copolymer nanodomain has been reported thus far using similar techniques. It would also be highly desirable to have a novel way of associating the metal (Co and/or Fe) to the polymer in the liquid state. Moreover, the specific reaction scheme for Co3O4 nanocomposites, where the Co atoms are directly attached to the monomer during its polymerization, is also desirable for obtaining ferromagnetic nanoparticles within a diblock copolymer matrix.